Alignment
The alignment process is one of making sure that your car "tracks" properly. That includes proper alignment of the front wheels and in certain cases, also the rear wheels. The entire process contributes to the ease of vehicle handling, overall safety and the prevention of premature tire wear. To explain this in the most basic form, a wheel alignment consists of adjusting the angles of the wheels so that they are perpendicular to the ground and parallel to each other. The purpose of these adjustments is maximum tire life and a vehicle that tracks straight and true when driving along a straight and level road. These are the details in simple presentation:
Alignment - Aligning a vehicle's front wheels is the job of balancing the vehicle's physical steering angles with the physical forces of nature being exerted upon the vehicle. The vehicle's steering angles are by definition caster, camber, toe-in, steering axis inclination, and toe-out on turns. The physical forces of nature are gravity, momentum, friction, and centrifugal force. Since so many factors are involved in front wheel alignment, it is also called often termed front end alignment, steering alignment, steering balance, or steering geometry; all are synonymous. Alignment is more than just adjusting the math angles of the front wheels. With steadily increasing production of front wheel drive vehicles equipped with independent rear suspension, four wheel alignment is often required. For ideal wheel alignment, certain conditions would have to be met. Both front tires will be the same brand, size, and type. Each will have the same degree of tread wear, and be inflated with the same pressure. Each wheel is properly and equally adjusted for correct angles, each tire will maintain the same area of tread contact on a smooth road surface. Obviously, it is impossible to maintain all these requirements all of the time. The steering control rods are used to adjust toe-in and toe-out. The control arms, upper and lower, are adjusted to correct angles of the camber angle. Caster is usually not adjustable on most vehicles. With all the weight balance factors to be checked out and corrected, it is obvious that wheel alignment is more than just an adjustment of the steering angles. The whole theory of wheel alignment revolves around balanced weight distribution on the wheels and proper tire tread contact with the road surface while the vehicle is in motion. The vehicles height above the ground is also important and can indicate visually a failure of one or more suspension components.
Definitions
Caster - When you turn your vehicle's steering wheel, the front wheels respond by turning on a mechanical pivot attached to the suspension system. Caster is the angle of this steering pivot, measured in degrees, when viewed from the side of the vehicle. These are some possible cause and effect conditions. If the top of the pivot is leaning toward the rear of the car, then the caster is positive, if it is leaning toward the front, it is negative. If the caster is out of adjustment, it can cause problems in straight line tracking. If the caster is different from side to side, the vehicle will pull to the side with the less positive caster. If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when you hit a bump. Caster has little affect on tire wear. The best way to visualize caster is to picture a shopping cart caster. The pivot of this type of caster, while not at an angle, intersects the ground ahead of the wheel contact patch. When the wheel is behind the pivot at the point where it contacts the ground, it is in positive caster. Picture yourself trying to push the cart and keep the wheel ahead of the pivot. The wheel will continually try to turn from straight ahead. That is what happens when a car has the caster set too far negative. Like camber, on many front wheel drive (FWD) vehicles, caster is not adjustable. If the caster is out on these cars, it indicates that something is worn or bent, possibly from an accident, and must be repaired or replaced.
Camber - This is the angle of the wheel, measured in degrees, when viewed from the front of the vehicle. If the top of the wheel is leaning out from the center of the car, then the camber is positive, if it's leaning in, then the camber is negative. Here are some cause and effect conditions. If the camber is out of adjustment, it will cause tire wear prematurely on one side of the tire's tread. If the camber is too far negative, for instance, then the tire will wear very prematurely on the inside of the tread. If the camber is different from side to side it can cause what is often termed a pulling problem where the vehicle tries to move to the outside or inside of the normal tracking path. The vehicle will pull to the side with the more positive camber. On many front wheel drive (FWD) vehicles, camber is not adjustable. If the camber is out on these cars, it indicates that something is worn or bent, possibly from an accident and must be repaired or replaced.
Steering Axis Inclination (SAI) - This is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. This angle, when added to the camber to form the (calculated but not measurable) included angle, causes the vehicle to lift slightly when you turn the wheel away from a straight ahead position. This action uses the weight of the vehicle to cause the steering wheel to return to the center when you let go of it after making a turn. Because of this, if the SAI is different from side to side, it will cause a side to side pull at very slow speeds. Most alignment machines have a way to measure SAI; however it is not separately adjustable. The most likely cause for SAI being out is bent parts which must be replaced to correct the condition. SAI is also commonly referred to as KPI (King Pin Inclination) on trucks and old cars that have king pins instead of ball joints.
Toe-in and Toe-out - The toe measurement is the difference in the distance between the front of the two tires and the back of the two tires, when as close to parallel as can be obtained. It is measured in fractions of an inch in the US (millimeters in Europe and Asia) and is usually set close to zero which means that the wheels are parallel (or as close as is possible) with each other. Toe-in means that the fronts of the tires are closer to each other than the rears of the tires. Toe-out is exactly the opposite. An incorrect toe adjustment will cause the tires to erase themselves in a scrubbing action. Incorrect toe-in will cause rapid tire wear to both tires equally. This type of tire wear is called a saw tooth wear pattern. If the sharp edges of the tread sections are pointing to the center of the car, then there is too much toe-in. If they are pointed to the outside of the car then there is too much toe-out. Toe is always adjustable on the front wheels and on some cars, is also adjustable for the rear wheels.
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